The present invention relates to an apparatus and method for determining the density of materials and, more particularly, relates to an apparatus and method for measuring the density of thin layers of materials.
Nuclear radiation gauges have been widely used for measuring the density of soil and asphaltic materials. Such gauges typically include a source of gamma radiation which directs gamma radiation into the test material, and a radiation detector located adjacent to the surface of the test material for detecting radiation scattered back to the surface. From this detector reading, a determination of the density of the material can be made.
These gauges are generally designed to operate either in a xe2x80x9cbackscatterxe2x80x9d mode or in both a backscatter mode and direct transmission mode. In gauges capable of direct transmission mode, the radiation source is vertically moveable from a backscatter position, where it resides within the gauge housing, to a series of direct transmission positions, where it is inserted into small holes or bores in the test specimen.
Many of the gauges commonly in use for measuring density of soil, asphalt and other materials are most effective in measuring densities of materials over depths of approximately 4-6 inches. However, with the increase in cost of paving materials, the practice in maintaining and resurfacing paved roadbeds has become one of applying relatively thin layers or overlays having a thickness of one to three inches. With layers of such a thickness range, many density gauges are ineffective for measuring the density of the overlay because the density reading obtained from such gauges reflects not only the density of the thin layer, but also the density of the underlying base material.
Nuclear gauges capable of measuring the density of thin layers of materials have been developed by the assignee of the present invention. For example, thin layer density gauges are disclosed in U.S. Pat. Nos. 4,525,854, 4,701,868, and 4,641,030, all of which are assigned to the assignee of the present invention and are incorporated herein by reference in their entirety. The gauges disclosed in the above-referenced patents are referred to as xe2x80x9cbackscatterxe2x80x9d gauges because the radiation source does not move outside the gauge housing, which is necessary for measurement in the direct transmission mode.
As disclosed in the above patents, the preferred method of measuring the density of thin layers of materials, such as asphalt, requires two independent density measurement systems. The geometry of these two measurement systems must be configured with respect to one another and with respect to the medium being measured in such a manner that they measure two different volumes of material. The two different volumes are not mutually exclusive insofar as they partially overlap one another. Measurement accuracy depends upon a larger portion of the volume measured by one of the measurement systems being distributed at a lower depth beneath the gauge than the volume measured by the other measurement system. This is accomplished by placing one radiation detection system in closer spatial proximity to the radiation source than the other detection system.
There remains a need in the art for a nuclear gauge capable of operating in both backscatter mode and direct transmission mode, and which is suitable for measuring the density of thin layers of material.
The present invention provides a nuclear density gauge capable of operating in both backscatter and direct transmission modes and also capable of accurately measuring the density of thin layers of materials. The nuclear gauge of the present invention minimizes the effect of variance in radiation source positioning on the accuracy of the density reading. The source rod of the nuclear gauge of the present invention has a maximum radial movement of less than about 0.003 inch at each predetermined source rod position and, preferably, a maximum radial movement of less than about 0.002 inch. Additionally, the source rod of the nuclear gauge of the present invention has a maximum vertical movement of less than about 0.003 inch at each predetermined source rod position and, preferably, a maximum vertical movement of less than about 0.002 inch.
The nuclear gauge of the present invention is suitable for measuring the density of the thin layer of material overlying a base material and comprises a gauge housing having a vertical cavity therethrough and a base. Within said housing, first and second radiation detectors are located, both detectors being positioned adjacent to the base of the gauge housing. The two radiation detectors are in separate positions within the gauge housing. The gauge further comprises a vertically moveable source rod extending into the cavity of the gauge housing. The source rod contains a radiation source within a distal end thereof. The gauge further comprises at least one bearing operatively positioned to guide the source rod within the vertical cavity of the gauge housing. The gauge also includes means for vertically extending and retracting the source rod to a plurality of predetermined source rod positions so as to change the spatial relationship between the radiation source and the two radiation detectors.
Preferably, the means for vertically extending and retracting the source rod includes an index rod operatively positioned adjacent to the source rod. The index rod has a plurality of notches, each of the notches corresponding to one of the predetermined source rod positions. The means for extending and retracting further comprises a handle affixed to the source rod. The handle includes a cavity therethrough and an indexer. The index rod extends into the cavity of the handle and the indexer is operatively positioned for engaging the notches of the index rod in order to temporarily affix the source rod in one of the predetermined positions. Preferably, at least two pins are used to affix the handle to the source rod. A spring is preferably used to bias the indexer into engagement with the notches of the index rod. For example, a spring having a spring rate of at least about 20 lbs. per inch may be used. In a preferred embodiment, the index rod has a substantially cylindrical shape extending from a distal end of the rod to the position of a notch of the index rod corresponding to the backscatter position.
The gauge also comprises a safety shield coaxially mounted around the vertical cavity of the gauge housing. The safety shield includes a bearing operatively positioned to guide the source rod through the vertical cavity.
Additionally, the nuclear gauge preferably comprises a radiation shield assembly operatively positioned to move laterally between two positions; a first position blocking a distal end of the vertical cavity of the gauge housing such that radiation is shielded from exiting the cavity and a second position adjacent to the vertical cavity and allowing vertical movement therethrough. In a preferred embodiment, the radiation shield assembly comprising a sliding block operatively positioned to move laterally between the first position and the second position, a spring engaging the sliding block and biasing the sliding block into the first position, and a fixed block. The fixed block preferably includes a track engaging the sliding block and guiding movement of the sliding block. Advantageously, a ball plunger engages the sliding block, and is operatively positioned to prevent vertical movement of the sliding block at the sliding block moves laterally between the first and second positions. A spring engages the ball plunger and biases the ball plunger towards the sliding block. This spring preferably has a spring rate of at least about 50 lbs. per inch.